This invention relates to a data transmission control system which is able to perform a circuit switching function in a token ring type local area network.
Heretofore, local area networks have been able to use only a packet switching method and there are no local area networks making use of a circuit switching method. Since there is no prior art in this respect, a packet switching method for a token ring system will be outlined below.
FIG. 19 schematically shows a token ring network system, which includes terminals 101a through 101c, terminal interface lines 102a through 102c, a unidirectional ring transmission line 103, and nodes 14a through 104c each capable of reproducing and repeating signals on the ring transmission line 103 and permitting data transmission between the terminal 101 and the ring transmission line 103. The data transmission to the terminal 101 is effected via the terminal interface line 102. A ring transmission line controller 105 is provided for token control and generation of a system clock to synchronize the entire system. The central controller 105 may be replaced by a local controller installed in each node 101.
FIG. 20 shows the data tranmission in the token ring network of FIG. 19. A token arrives at the node 104a at a time to. At this time, the node 104a has data to be transmitted and sends it out as a frame A to the ring transmission line 103 while reproducing the token immediately after the frame A at a time t.sub.1. The node 104b has not data to be transmitted and merely reproduces and repeats the frame A and the token at a time t.sub.2. If the frame A is addressed to the node 104b, the node 104b receives the frame A while performing the reproduction and repeating. At a time t.sub.3, the node 103c transmits a frame C and reproduces the token, while the frame A is reproduced and repeated by the controller 105. At a time t.sub.4, the node 104a discards the frame A which has completed a cycle of excursion through the ring transmission line 103, and receives the token again. In this example, the token excursion time at the node 104a is t.sub.4 - t.sub.0 ).
In this token ring network, the token excursion time is the sum of the propagation time through the ring transmission line 103 including the reproduction and repeating time at each node and the time length of a frame transmitted including the time length of a token. Consequently, it varies widely depending on the size of a system and the transmission status of each node. Because of this variation, the token ring system networks, which have been able to use a packet switching method, have been unable to employ a circuit switching method.
In order to use a circuit switching method, it is necessary to maintain a constant speed or rate of data transmission between terminals. The variation in the token excursion time, however, presents the following problems. If the data transmission rate between terminals is X bits/sec (constant) and the token Excursion time is t sec (variable), then the transmnitter data buffer stores X.multidot.t bits of new data and transmits them to the receiver data buffer via the ring transmission line 3. The data received by the receiving buffer is transmitted to the receiving terminal at the constant rate of X bits/sec so that X.multidot.t bits of data are transmitted to the receiving terminal in t seconds. Consequently, if the next data does not arrive at the receiving buffer in t seconds, an underlfow condition takes place in the receiving buffer, resulting in a temporary interruption of data transmission to the terminal. In the token ring system, the token excursion time always varies, and such an underflow condition frequently occurs unless some countermeasure is taken. Otherwise, it is impossible to implement a circuit switching method in a token ring network.